Of the various types of valves utilized for controlling the flow of fluid within conduit systems, rotary plug valves have been found to be quite popular. Rotary plug valves are typically of compact, low profile design and yet offer much the same fluid flow dimension that is provided by many other types of valves having much larger physical dimensions. Where physical size and height is an important or mandatory design consideration, rotary plug valves are widely specified. Rotary plug valves are typically of simple nature, utilizing few moving parts and are of nominal cost. These various advantageous features substantially enhance the competitive nature of plug valves.
Among the disadvantages of rotary plug valves, however, are the disadvantages of operational pressure range limitations and limitations from the standpoint of sealing capability. Most rotary plug valves function properly only at relatively low pressure ranges, for example, in the order of 0-150 psig. This operational pressure restriction, in many flow systems, requires that other less desirable, but higher pressure valves be utilized. Where plug valves incorporate lubricant enhanced sealing, additional sealant must be introduced into the sealant chambers quite often to ensure against leakage. This requirement is an expensive service consideration which is frequently detrimental to the employment of such valves. Also, the lubricant sealant must be compatible with the product controlled by the valve, thus ensuring against contamination of the product.
In many cases, elastomeric sealing material is incorporated in plug valve mechanisms for establishment of a seal between the rotatable plug and the sealing surfaces of the valve body. Elastomeric sealing material, however, tends to wear at a fairly rapid rate, especially in valves of larger size where the sealing elements slide a considerable distance in contact with the internal sealing surfaces of the valve. Rapid wear of this nature presents a problem from the standpoint of servicing, especially if the valve is intended for frequent cycling during use. The sealing elements of plug valves can also become damaged when opened or closed under high pressure or high velocity service conditions. For example, during 90.degree. rotation of the plug element, the sealing elements must move past the various ports defined by intersection of the flow passages of the valve with the valve chamber. When this activity occurs, the violent activity that is created by the flowing fluid medium can partially extrude or otherwise force the sealing material of the seals from the seal grooves. Rotation of the plug member while in sealing contact with the valve body can cause the sealing elements to be cut or otherwise damaged as they slide past the valve ports, thus requiring replacement of the seals. To prevent excessive wear or damage of the seals of tapered plug valves, it is desirable to actuate such valves between open and closed positions thereof by unseating, rotational and reseating increments of valve movement. Unseating of tapered plugs is accomplished by moving the plug linearly until the seals of the plug are moved clear of corresponding seat surfaces of the valve. The plug is then rotated to a selected position, being free of any contact with the seat surface during rotation. After being properly positioned, the plug is then moved linearly in the opposite direction to again bring the sealing elements thereof into seating contact with the seat surface.